It is often desirable to focus or concentrate particles in a sample environment by forming a particle beam. For example, the detection of the number or presence of particles may employ the formation of a particle beam. Such particle beam formation and subsequent detection is desirable, for example, during microelectronic device processing in which the presence of particles is detrimental to the performance of the resulting microelectronic devices because the particles may deposit on the microelectronic devices' surface and lead to defects which reduce the device yield. By detecting the particles' presence, preventative or corrective measures may be taken to salvage the microelectronic devices.
Several systems have been proposed to focus a particle beam derived from an aerosol in which the particles are suspended in a gas. Such systems typically include a nozzle through which the aerosol is drawn to generate and focus the particle beam. After passing through the nozzle, the gas, whose mass is substantially less than that of the heavier particles, diverges laterally rapidly while the particles may first converge to a focal point prior to slowly diverging laterally. Typical particle beam generation devices direct the beam of slowly diverging particles and rapidly diverging gas to a skimmer having an opening therethrough. The skimmer allows the beam, now consisting of the majority of the particles but only a small fraction of the gas which passed through the nozzle due to the relative rates of divergence of the gas and particles, to pass while redirecting the gas. The particle beam passing through the skimmer generally enters a chamber having a relatively low pressure for detection.
There are three principal types of nozzles utilized to focus aerosol beams. A first such nozzle is a converging conical nozzle such as those described in publications entitled Properties of Continuum Source Particle Beams by Dahneke et al., Journal of Aerosol Science, Vol. 10, 1979, pp. 257-274; and Aerodynamic Focusing of Particles in a Carrier Gas by Fernandez de la Mora et al., Journal of Fluid Mechanics, Vol. 195, 1988, pp. 1-21. A second type of typical nozzle is a capillary tube such as those disclosed in publications entitled Mass Distribution of Chemical Species in a Polydisperse Aerosol: Measurement of Sodium Chloride in Particles by Mass Spectrometry by Sinha et al., Journal of Colloid and Interface Science, Vol. 112, No. 2, August 1986, pp. 573-582; Particle Analysis by Mass Spectrometry by Sinha et al., Journal of Colloid and Interface Science, Vol. 87, No. 1, May 1982, pp. 140-153; and On the Real-Time Measurement of Particles in Air by Direct-Inlet Surface-Ionization Mass Spectrometry, by Stoffels et al., Vol. 40, 1981, pp. 243-254. A third type of nozzle is a thin plate positioned substantially orthogonal to the longitudinal direction of propagation of the aerosol beam. The thin plate has an orifice therein as described in the publication entitled Mass Spectrometric Analyzer for Individual Aerosol Particles by Allen et al., Review of Scientific Instruments, Vol. 52, No. 6, June 1981, pp. 804-809 and Aerodynamic Focusing of Particles and Molecules in Seeded Supersonic Jets by Fernandez de la Mora, et al., Rarefied Gas Dynamics: Physical Phenomena, Vol. 117 of Progress in Astronautics and Aeronautics, 1989, pp. 247-277.
While all three typical nozzle designs focus the aerosol beam to some extent, the amount of control over the shape of the beam, such as its rate of convergence or divergence or its focal length, is limited. For example, capillary tubes do not produce focused beams and thin plates having an orifice therein produce beams having very short focal lengths which diverge rapidly after passing through the focal point. Furthermore, to the best of the inventors' knowledge, previous investigations of nozzles used to produce particle beams sampled aerosols at pressures ranging from atmospheric to about 20 torr while the particles' diameter was always larger than about 0.1 um.
In order to obtain increased control over the shape of a resultant aerosol beam, nozzles incorporating a capillary tube immediately adjacent to and in contact with a thin plate having an orifice therethrough have been utilized. Such nozzles consisting of a capillary tube and a thin plate having an orifice are described in publications entitled Measurement of Externally Mixed Sodium Containing Particles in Ambient Air by Single Particle Mass Spectrometry by Giggy et al., Atmospheric Environment, Vol. 23, No. 10, 1989, pp. 2223-2229; and Electron-Impact Ionization Time-Of-Flight Mass Spectrometer for Molecular Beams by Pollard et al., Review of Scientific Instruments, Vol. 58, No. 1, January 1987, pp. 32-37. While the nozzles described in the Giggy and Pollard articles may improve, somewhat, the control over the aerosol beam's shape, the amount of control over the beam's convergence or divergence or its focal length may be limited.
An alternative device to provide increased control over the shape of a particle beam is described in a publication entitled Aerodynamic Focusing of Particles in Incompressible Jets by Rao et al., IBM Publication No. IBM-EF-23, Apr. 9, 1992; and U.S. Pat. No. 3,854,321 to Dahneke which issued on Dec. 17, 1974 (hereinafter the '321 patent). As illustrated in FIGS. 1, 3 and 4 of the '321 patent, sheath air is introduced about the aerosol beam, prior to its focusing by the nozzle, to constrain the incident aerosol beam. By using sheath air directed substantially parallel to the flow of the aerosol beam, the aerosol beam, both the particles and the gas, diverges less from its longitudinal axis of propagation. The use of sheath, however, requires a means to produce and to control the flow of the sheath air and adds to the pumping requirements of the system.
Thus, although the formation of particle beams is highly desirable, particularly for use in conjunction with particle detection devices as illustrated in a publication entitled Secondary Electron Emission From Beams of Polystyrene Latex Spheres by Hall et al., Journal of Applied Physics, Vol. 47, No. 12, December 1976, pp. 5222-5225, to the best of the inventors' knowledge the art has not heretofore suggested a viable method and apparatus for providing adequate control over the shape and focal length of a particle beam, particularly at relatively low pressures and for relatively small particles. In particular, to the best of the inventors' knowledge, prior particle beam generation devices which produced particle beams of a known and controlled shape were not used to produce particle beams from aerosol sources having pressures less than 20 torr and particles smaller than 0.1 um in diameter.